A variety of emissions, such as nitrogen oxides (e.g., NO and NO2), are emitted in exhaust gases of internal combustion engines. In order to decrease emissions from motor vehicles, emissions are regulated via use of exhaust system components, such as catalytic converters. Additionally, various gas sensors, including NOx sensors, are employed to detect the emissions in exhaust gases.
Minimizing engine out emissions involves a balance of oxidizing and reducing exhaust constituents such as carbon monoxide (CO), soot, unburnt hydrocarbons (HC), and NOx. Diesel engines with compression ignition operate under lean air-fuel ratios most often, and therefore can have high levels of NOx production compared to gasoline engines. To reduce the production of NOx, exhaust gas recirculation (EGR) systems may be utilized to divert a portion of the exhaust gas back to the intake, thereby reducing peak combustion temperatures and pressures so as to lower NOx.
However, under these conditions, soot or other particulate matter production can increase. Additionally, due to the lowered oxygen concentration in the cylinder charge when EGR is present, under certain conditions, combustion may become unstable e.g., knocking and/or misfiring may occur, which can degrade the engine and lead to reduced fuel economy.
The inventors have recognized the above issues and offer a method to at least partly address them. In one embodiment, a method for controlling combustion in an engine comprises under a first condition, adjusting an EGR amount of a total cylinder charge in response to engine out NOx levels being below a first threshold.
In this way, engine out NOx levels may be used as feedback to control combustion stability. In one example, if NOx levels decrease below a threshold level, it may indicate a degraded combustion condition, for example, due to a higher than optimal EGR percentage of the cylinder charge. To compensate, the EGR percentage may be decreased by adjusting an EGR valve.
The present disclosure may offer several advantages. In one embodiment, by detecting degraded combustion via NOx feedback, conditions that reduce fuel economy and increase soot production may be detected, and a delivered EGR amount may be adjusted to compensate. Thus, fuel economy may be improved, and soot emissions may be reduced. Further, NOx levels may be maintained within a desired window such that if NOx levels are too high, additional EGR may be provided to lower NOx emissions.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.